Chemistry decay question

how can this be D? i thought even with decay they only loose their own properties not change into other substances?

In beta decay, 1 neutron changes into 1 proton and 1 electron. Electron is released from the atom, but proton stays in the nucleus, therefore increases the atomic number (number of protons). Thus, cobalt changes into nickel because its atomic number increases by 1. Gamma radiation is just gamma waves emitted by the nucleus and it does not change anything about the atom.